A number of physical deposition methods are known for coating workpieces, in particular tribologically heavily stressed components that are simultaneously subject to an additional stress of a different type, such as temperature or cavitation. Two different methods from the field of plasma coating technology for producing carbon coatings that are free of metal and hydrogen (so-called tetrahedrally coordinated amorphous carbon, ta-C) can be named: deposition of graphite targets by sputtering or by arc vaporization.
The sputtering method is characterized by a low deposition rate, however, so that this method is poorly suited for economical use.
In arc vaporization the cathode is typically made of the material to be vaporized, while a special electrode is available as the anode, or the wall of the coating chamber can function as the anode. To vaporize the target material, an arc is ignited between the anode and the cathode which heats the cathode locally so intensely at the arc spot, recognizable by a burning point on the cathode, that the target material changes to the vapor phase and precipitates on a workpiece. An ignition device is needed to produce the arc between the anode and the cathode.
In addition to movable ignition devices, fixed ignition electrodes located near the target cathode are known, between which an arc is ignited which then moves from the ignition electrode to the anode.
Controlled direct current (dc) arc vaporization of carbon turns out to be difficult, since the burning point of the arc tends to become stuck at one place on the target and possibly to burn through. It is also known that in the vaporization process droplets (macro-particles) result in increased roughness of the coating on the workpiece. This method is therefore also only employed to a limited extent.
With pulsed arc discharge, on the other hand, pulsing current is applied between the anode and the cathode, causing the arc spot on the target to move approximately 100 times as fast as in dc arc vaporization, thereby preventing the localized burning. The pulsed arc discharge generally has a pulse length in the range of milliseconds (msec), which causes the discharge to be localized in spatial proximity to the ignition.
If one wishes to utilize the technology of pulsed arc discharge with large-area targets, an arrangement using a plurality of individual ignition sources therefore makes sense.
From Russian Patent No. RU 2153782 a carbon plasma pulse source for applying a carbon coating to a workpiece is known, which includes among other components a graphite cathode, an anode, a capacitive accumulator circuit and at least two ignition units positioned on the circumference of the graphite cathode. With this system the area coated with the layer of carbon is enlarged compared to a device having only one ignition unit, and at the same time the thickness of the layer applied to the workpiece is more uniform. The ignition units include a rod-shaped metallic electrode and a ring-shaped graphite electrode which function as the ignition cathode and the ignition anode, respectively. Positioned between the ignition cathode and the ignition anode is a ring made of a dielectric material, the dielectric material being coated on the side facing the target cathode with an electrically conductive material. The longitudinal axis of each ignition unit is directed towards the corresponding region of the working area of the target cathode which is provided for starting the arc discharge.
The ring-shaped structure of the ignition cathode and of the dielectric material may have a detrimental effect on economic and maximally homogeneous utilization of the target surface: The quasi-punctiform action of the ignition source causes the locus of ignition on the target surface to be confined to a relatively small zone, so that a certain place is eroded from repeated discharges, while the adjacent areas on the target surface remain largely unaffected. In time, such pronounced erosion zones result in undesired cratering on the target cathode.
In order to be able nevertheless to erode the entire target surface as homogeneously as possible, the ignition sources must be located as close together as possible. This results in an increase in the number of ignition sources needed per unit of length of the target cathode, and consequently in an increase in the maintenance expense and in instability when the coating system is used continuously.
It must also be kept in mind that the ignition units are subject to severe heat, and should be cooled for stable long-term operation and/or high ignition frequencies. The cooling of round-shaped ignition units is quite complex technologically, since for each individual ignition unit the cooling device must be adapted to the ring-shaped structure of the electrodes, and therefore should have corresponding roundings.
Overall, the described problems make it difficult to make simple and stable use of the coating method using pulsed vacuum arc discharge economically with large-surface targets.